[Bug-gsl] [bug #43256] gsl_sf_coupling_6j overflows

[Anders Søndergaard]

URL:
  <http://savannah.gnu.org/bugs/?43256>

                 Summary: gsl_sf_coupling_6j overflows
                 Project: GNU Scientific Library
            Submitted by: andersas
            Submitted on: Fri 19 Sep 2014 12:45:52 PM GMT
                Category: Runtime error
                Severity: 3 - Normal
        Operating System: Ubuntu linux
                  Status: None
             Assigned to: None
             Open/Closed: Open
                 Release: 1.16 (ubuntu latest)
         Discussion Lock: Any

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Details:

I need to calculate 6-j symbols for somewhat large J.
For example the symbol: 

(14, 16, 16
 78, 62, 76) = 6/1739*sqrt(16971005238954/1556382731177197) = 0.000360286

but this fails because gsl_sf_fact overflows.
It is used internally in gsl_sf_coupling_6j.


I have looked a bit at the code, and I think in these large
cases it is possible to work with the logarithm of the factorials
and in the end exponentiate.

Other libraries I have tested also gets these symbols wrong (but silently
returns the wrong value),
except pythons sympy, which calculates the symbols as the square root of a
rational number.

I would rather not interface my Fortran program with Python.






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